Thanks for fast reply! I do not use any SIP device. The call is coming from another SIP service and there is following dial plan involved:

if sys.In then
CID = req.Header.From.FromURI.User;
sys.Log("New incoming call received to #{req.URI.User} from #{CID}.")
sys.dial("PSTN_PHONE_NUMBER@provider1");
end

if I use provider1 with same credentials in ZOIPER SIP client on my iPhone, it is working just fine.
if I use some another voip provider as provider1 (another SIP provider I use) it is also working.
So there is something with my provider1 setting in "SIP Providers" I used just username, sip server and password (I did not fill any additional optional SIP provider settings options
Thanks again.

I found out it is not working only if I call from tropo.com service (call transferred from tropo dialplan)
Does anybody know how to make tropo transferred call to supply body when transferred to sipsorcery? Thanks.